A strange case of a woman with a hand amputation lends credence to the idea that our brains know what our bodies should be like, even if our bodies vary from the norm. The woman, referred to as RN, was born with a deformed right hand that had only three fingers. That's all she ever had. Then when she was 18, her hand was amputated after an auto accident. Like many amputees, she eventually began to feel she had a "phantom hand."

"But here's the interesting thing," says Paul McGeoch at the University of California, San Diego. "Her phantom hand didn't have three digits, it had five."

RN was aware of a full complement of fingers, but her phantom thumb and index finger were less than half the usual length.

With training using a mirror box trick – a tool that creates the visual illusion of two hands – McGeoch and V.S Ramachandran, also at San Diego, managed to extend her short phantom finger and thumb to normal length.

McGeoch says this study indicates that there is a hardwired representation in the brain of what the body should look like, regardless of how it actually appears in real life. It shows us more about the balance between the external and innate representations of a limb, he says.

Well, if you're going to imagine a hand, you may as well imagine one that matches your other one. She should have imagined something awesomer, though, like a giant robot claw/arc welder/machine gun/rocket launcher.